1. Field of the Invention
The present invention generally relates to liquid crystal display units having wider viewing angles.
2. Description of the Related Art
FIG. 16 illustrates an example of the configuration of a liquid crystal display unit. In the liquid crystal display unit H shown in FIG. 16, a twisted nematic (TN) liquid crystal 3 is disposed between opposing lower and upper substrates 1 and 2, and polarizers 4 and 5 are placed on the exterior of the upper and lower substrates 2 and 1, respectively. A thin-film-transistor-type liquid crystal display unit H is configured as follows. A source wiring pattern 6 and a gate wiring pattern 7 are provided in a matrix form on the lower substrate 1. A thin film transistor 8 and a pixel electrode 9, both of which serve as a switching device, are provided in an area where the source wiring pattern 6 and the gate wiring pattern 7 intersect with each other. Moreover, a color filter 10 and a common electrode 11 are disposed on the upper substrate 2. A potential difference is provided between the pixel electrode 9 and the corresponding region of the common electrode 11 in each matrix area so as to apply a certain electric field to the liquid crystal 3 between the two electrodes. The alignment of the liquid crystal 3 is thus controlled.
Further, in the liquid crystal display unit H of the above type, an alignment film (not shown) is provided on the upper surface of the lower substrate 1 next to the liquid crystal 3 and on the lower surface of the upper substrate 2 next to the liquid crystal 3 so as to control the alignment of the liquid crystal molecules when an electric field is not applied. The alignment film provided on the lower substrate 1 shown in FIG. 16 is rubbed in the direction indicated by the arrow B shown in FIG. 17, while the alignment film disposed on the upper substrate 2 illustrated in FIG. 16 is rubbed in the direction indicated by the arrow A illustrated in FIG. 17.
Accordingly, in the liquid crystal display unit H shown in FIG. 16, when an electric field is not applied to the liquid crystal 3, the liquid crystal molecules which are homogeneously aligned between and parallel to the substrates 1 and 2 are twisted through 90xc2x0. On the other hand, when an electric field is applied to the liquid crystal 3, the liquid crystal molecules are aligned between and perpendicularly to the substrates 1 and 2 in the direction along the electric field. Thus, the liquid crystal display unit H is switched between a bright state and a dark state by alternately allowing light emitted from a backlight in the direction from below the lower substrate 1 to pass through the liquid crystal display unit H or blocking it.
However, the TN-mode liquid crystal display unit of the above type presents the problem of the viewing angle dependency. FIG. 18 illustrates a typical viewing angle dependency of the TN-mode liquid crystal display unit, and the hatched portion indicates an area having a contrast (CR) of 10 or greater. FIG. 18 reveals that the TN-mode liquid crystal display unit exhibits a poor visibility as viewed from the vertical (up and down) direction, in particular, from above, though the visibility in the horizontal (left and right) direction is satisfactory.
In the above background, various structures for increasing the viewing angles of a liquid crystal have been proposed.
Hitherto, in a technique for widening the viewing angles of the above type of liquid crystal display unit, the following structure is known in which the orientations of alignment of the liquid crystal molecules are differentiated between a plurality of regions of each pixel. In this structure, each pixel has a plurality of domains (areas) where the liquid crystal molecules are aligned perpendicularly to the substrates in different orientations when a voltage is applied to the molecules. In this structure, each pixel is usually divided into two domains, and the alignment film portions corresponding to the divided two domains are processed for alignment of the liquid crystal in the different orientations. By virtue of this structure, a sharp and asymmetrical change in the contrast in the vertical direction, which is conventionally encountered by the TN-mode liquid crystal display unit, is alleviated and then becomes symmetrical, thereby inhibiting the inversion of halftones. It is thus expected that a liquid crystal display unit having wider viewing angles will be provided.
In order to implement this structure, however, it is necessary that an alignment film, which is used for controlling the alignment of the liquid crystal, be divided into a plurality of very small areas for each miniscule pixel, and that the divided areas of the alignment film be processed to effect the different orientations of the alignment of the liquid crystal. It is, however, very difficult to process the divided areas of the alignment film in accordance with the divided pixel regions, thereby lowering the yield and increasing the complexity of the manufacturing process.
Accordingly, in view of the above background, it is an object of the present invention to provide a liquid crystal display unit which exhibits wider viewing angles and a brighter display, and in which the need for processing alignment film for the alignment of a liquid crystal is eliminated so as to simplify the manufacturing process.
In order to achieve the above object, according to the present invention, there is provided a liquid crystal display unit including a first substrate and a second substrate disposed in such a manner that they face each other. A liquid crystal having a negative anisotropy of dielectric constant is disposed between the first and second substrates. A common electrode and an alignment film which has a pretilt angle of 90xc2x0xc2x11xc2x0 and which is not rubbed are sequentially disposed on the surface of the first substrate facing the second substrate. A plurality of pixel electrodes are disposed on the surface of the second substrate facing the first substrate so as to cover a display area of the liquid crystal. A conductive light-shielding member is disposed on the second substrate and is positioned around each of the plurality of pixel electrodes in a non-display area of the liquid crystal while being electrically insulated from the pixel electrode. The conductive light-shielding member is set at substantially the same potential the said common electrode. An alignment film which has a pretilt angle of 90xc2x0xc2x11xc2x0 and which is not rubbed disposed on the pixel electrodes and on the conductive light-shielding members.
In the foregoing liquid crystal display unit, an optical compensator may be provided for at least one of the first and second substrates.
As discussed above, the liquid crystal having a negative anisotropy of dielectric constant is disposed between alignment films which have a pretilt angle of 90xc2x0xc2x11xc2x0 and which are not rubbed (which are not processed for alignment of the liquid crystal). Accordingly, when an electric field is not applied, the long axis of the,liquid crystal is aligned perpendicularly to the alignment films. In contrast, upon application of an electric field generated from pixel electrodes, the long axis of the liquid crystal is homogeneously aligned between the alignment films. Since the liquid crystal molecules are positioned perpendicularly to the alignment films which are not particularly processed for alignment for the liquid crystal, they are not tilted in a specific direction when they are homogeneously aligned between the alignment films. Thus, the liquid crystal molecules are tilted in any given direction when they are homogeneously aligned upon application of an electric field, thereby resulting in the formation of a plurality of domains. As a consequence, the liquid crystal molecules are aligned in specific orientations by being tilted in any given directions upon application of an electric field.
Moreover, the liquid crystal has a negative dielectric constant, and the conductive light-shielding member is set substantially at the same potential as the common electrode. Accordingly, the electric lines of force produced by an electric field generated from the pixel electrodes are extended around the pixel electrodes while being attracted to the conductive light-shielding member provided around the pixel electrodes. Thus, the liquid crystal molecules are aligned perpendicularly to the extended electric lines of force, resulting in the formation of a plurality of domains for each pixel. The liquid crystal molecules are homogeneously aligned while having slightly different tilted angles for the individual domains.
Therefore, a sharp and asymmetrical change in the contrast in the vertical direction of the liquid crystal display unit is reliably eased and then becomes symmetrical, thereby suppressing the inversion of halftones.
The light-shielding member is grounded, thereby making it possible to divide each pixel into a plurality of domains without needing to divide an alignment film into a plurality of portions corresponding to the divided domains. Hence, the above advantages of the present invention are obtained without particularly processing alignment films for alignment of the liquid crystal. The liquid crystal display unit of the present invention can thus be manufactured more easily than the conventional ones having the above-described structure, thereby simplifying the manufacturing process.